Venturi tube flow meters are a species of differential pressure flow meters that infer a rate of a fluid flow in a closed piping system from a measurement of differential pressure that is caused by a change of kinetic energy in the fluid. The pressure drop across the flow meter is proportional to the square of the flow rate. The change of kinetic energy in a flowing fluid can be created by various types of flow measurement devices such as for example, orifice plates, pitot tubes, flow nozzles, wedge meters and venturi tubes.
Venturi tubes have the advantage of being able to handle large flow volumes while creating only low permanent pressure loss in the system. A venturi tube is essentially a section of pipe having a tapered upstream section that converges into a smaller diameter throat that is contiguous to a diverging downstream portion. All of these components are symmetrically disposed around the longitudinal axis of the conduit in which the fluid whose rate is to be measured is flowing. As fluid passes from the converging section through the throat, its velocity increases, causing a pressure differential between the venturi inlet and its throat.
Venturi flow meters, as described, are old in the art of flow measurement but with all of its advantages it, like other types of differential pressure flow meters, has disadvantages. One such disadvantage is the propensity of the changing profiles of the venturi tube to precipitate and collect sediment or solids from fluids having entrained solid materials, such as slurries and sanitary flows. Accumulation of this sediment, especially in the profile changes along the bottom interior surfaces of the venturi tube, clogs the flow meter and degrades its performance by upsetting its calibration.
Accordingly, it is the primary purpose of the present invention to provide an improved venturi flow meter that is not subject to entrapping and collecting sediment and other solid materials along and over its interior bottom surfaces, thus maintaining the calibration of the flow meter and ensuring its known accuracy, even with fluids containing suspended solids.
A further object of the invention is to provide a venturi flow meter with a smooth even bottom surface that is void of vertical profile angles and abrupt curves. Because the venturi tube requires a reduced diameter throat portion for its operation, the goal of having an unbroken regular bottom surface requires that the axis of the throat be offset from the axis of the conduit in which the fluid to be measured is flowing. Thus, the axis of the throat in the venturi tube of the present invention is eccentric to the axis of the fluid conduit.
The prior art has seen so-called eccentric venturi tubes, but the eccentric arrangement of the prior art devices does not lend itself to the purposes or objects described for the present invention. Accordingly, their structural limitations could not support their use as a flow measurement instrument. Such prior art devices may be seen in the disclosures of U.S. Pat. No. 4,325,460 for Ejector Muffler and U.S. Pat. No. 6,109,293 for Split Venturi, Axially Rotated Valve. Neither of the eccentric venturi devices included in the disclosure of these patents are concerned with fluid flow measurement, differential pressure creation or the avoidance of solid deposits.